1. Field
This application relates generally to scintillator devices, and more specifically, to techniques for hermetically sealing the scintillator material of an X-ray detector.
2. Description of the Related Art
An X-ray detector commonly comprises three key components: a scintillator, a photodiode array, and a readout circuit. The scintillator converts X-ray photons to visible photons that are detected by the photodiode array. The photodiode array then converts the visible photons to an electrical signal, which is read out by the readout circuit. The readout circuit may include thin film transistors (TFTs) that address the columns and rows of the photodiode array.
A common material used for the scintillator is cesium iodide (CsI) doped with thallium (Tl) due to its high absorption coefficient and high conversion efficiency for X-ray photons. CsI can be grown by vapor deposition in needle-like shapes (e.g., columns) on a substrate. The photodiode array and the readout circuitry (e.g., TFTs) may be patterned on the substrate on which the scintillator is formed. This configuration is commonly used for amorphous silicon (a-Si) detectors. For detectors made from a complementary metal oxide semiconductor (CMOS), the scintillator may be formed on a fiber optic plate (FOP) that is coupled to the CMOS sensor. Regardless of the substrate used, the columnar CsI structure decreases the lateral spreading of visible light due to total internal reflection at the column boundaries, thus maintaining high spatial resolution of the scintillator.
A disadvantage of using CsI for the scintillator, however, is that it is a hygroscopic material that tends to absorb moisture from the air and must be protected by a hermetic seal from the surrounding environment. Moisture can damage the structure of the CsI crystals, thereby degrading the image quality of the detector. One solution is to cover the entire CsI surface with a flat sheet and then seal the edges of the sheet to the substrate with epoxy or other organic sealants. Devices embodying this technique are described, for example, in U.S. Pat. Nos. 5,707,880 and 6,642,524.
A composite of aluminum (Al) and carbon fiber is a desirable material for the flat protective sheet because it absorbs a low amount of X-ray radiation and is both light weight and impervious to moisture. The sealant (e.g., epoxy), however, is not completely impervious to moisture, and moisture diffusion through the seal can cause the performance of the scintillator to degrade slowly over time.
FIG. 1 depicts another known device 100 for hermetically sealing a scintillator that incorporates aspects similar to those described in U.S. Pat. Nos. 6,414,316 and 8,415,628. Device 100 includes a substrate 102, scintillator layer 104, reflector layer 106 (e.g., Opticlad™), and a cover 108. Cover 108 is a rigid structure with a lip around its edge that is adhered to substrate 102 with epoxy 110. The lip of cover 108 extends towards substrate 102, reducing the cross-section of epoxy 110 as compared to the epoxy required for a flat sheet. The reduced cross-section of epoxy 110 may decrease the rate of moisture diffusion through the seal.
Although the lip of cover 108 improves the performance of the seal as compared to a flat sheet, there still remains an opportunity to better protect the scintillator from moisture, improve processes for manufacturing a scintillator device, and reduce the associated costs.